Aon Benfield has estimated this year’s record-breaking tornado season and severe weather in the US have caused $US15.5 billion ($14.8 billion) of insured losses, three times the average rate of the past 10 years.

Impact Forecasting, Aon’s catastrophe model development centre, says the largest tornado outbreak in recorded history saw five events during April and May that each caused losses of more than $US1 billion ($957 million). Total economic losses over April-May are estimated at $US21.65 billion ($20.7 billion).

Americans are asking why there was such a high death toll – 585 people – and Impact’s analysts point out that the US population has more than doubled since 1950 and there is more potential for tornadoes to cause fatalities when more people are living in urban areas.

Insured losses from the tornado season have averaged $US5.1 billion ($4.88 billion) a year for the past 10 years, and around 56 deaths a year are attributed to the events.

Impact Forecasting President Steve Jakubowski says the EF-5 (highest level) tornado that destroyed Joplin, Missouri, in late May caused 154 deaths in the city “becoming the deadliest singular tornado event since the National Weather Service officially began keeping records in 1950”.

He says the late April period saw the largest tornado outbreak ever recorded with 334 separate tornado touchdowns and catastrophic damage throughout the southeast and the Tennessee Valley.